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          <h1 class="post-title" itemprop="name headline">tcpdump抓包tcp三次握手四次挥手</h1>
        

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        <p>最近项目遇到了经典的<a href="https://www.google.co.jp/search?q=tcp+40ms+delay&amp;oq=tcp+40ms+delay&amp;aqs=chrome..69i57j69i60j69i59.240j0j9&amp;sourceid=chrome&amp;ie=UTF-8" target="_blank" rel="external">tcp 40ms delay</a>,tcpdump抓包显示当连续两次调用write操作时，第二次会等第一次接收到ack后才能发送给server端。</p>
<p>今天使用<code>tcpdump</code>抓包验证一下<code>tcp</code>连接的三次握手和四次挥手</p>
<h3 id="结论"><a href="#结论" class="headerlink" title="结论"></a>结论</h3><p>可能还需要进一步论证<br>TCP三次握手四次挥手可能描述上有些问题，下面实验表明可能存在着4次握手，三次挥手的情况，根据系统的不同（可能是网络参数优化造成的，最可能的是tcp_delayed_ack造成的)。 至于经典的<code>TCP 40ms delay</code>问题，是因为client端使用了<a href="https://en.wikipedia.org/wiki/Nagle&#39;s_algorithm" target="_blank" rel="external"><code>Nagle&#39;s_algorithm</code></a>优化网络，server端使用了<a href="https://en.wikipedia.org/wiki/TCP_delayed_acknowledgment" target="_blank" rel="external"><code>TCP DELAYED ACK</code></a>优化网络。具体原因和解决方案，可以看书籍<a href="https://book.douban.com/subject/1500149/" target="_blank" rel="external"><code>UNIX网络编程</code></a>第七章中关于TCP_NODELAY套接字的部分。</p>
<h3 id="测试一"><a href="#测试一" class="headerlink" title="测试一"></a>测试一</h3><ul>
<li>环境<ul>
<li>client server 同台机器</li>
<li>MacOS 10.13.1</li>
<li>原理使用<a href="https://www.google.co.jp/search?q=linux+nc&amp;oq=linux+nc&amp;aqs=chrome..69i57j0j69i61l2j69i59j0.357j0j9&amp;sourceid=chrome&amp;ie=UTF-8" target="_blank" rel="external"><code>nc</code></a>命令,模拟Server端和Client端</li>
</ul>
</li>
<li><p>执行</p>
<ul>
<li>1：终端执行 nc -l 9999 (模拟Server)</li>
<li>2：终端执行 tcpdump ‘port 9999’ -i lo -S (注意要找到ip为127.0.0.1对应的网卡)</li>
<li><p>3：终端执行nc localhost 9999 (模拟Client)</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div></pre></td><td class="code"><pre><div class="line">client向server发送SYN和seq</div><div class="line">15:55:29.810229 IP localhost.56498 &gt; localhost.distinct: Flags 		[S], 	seq 101048875, win 65535, options [mss 16344,nop,wscale 		5,nop,nop,TS val 726072074 ecr 0,sackOK,eol], length 0</div><div class="line"></div><div class="line">server向client发送SYN+ACK和seq</div><div class="line">15:55:29.810325 IP localhost.distinct &gt; localhost.56498: Flags 		[S.], seq 841661562, ack 101048876, win 65535, options [mss 		16344,nop,wscale 5,nop,nop,TS val 726072074 ecr 		726072074,sackOK,eol], length 0</div><div class="line"></div><div class="line">client向server发送ACK</div><div class="line">15:55:29.810341 IP localhost.56498 &gt; localhost.distinct: Flags 		[.], 	ack 841661563, win 12759, options [nop,nop,TS val 		726072074 ecr 	726072074], length 0</div><div class="line">	</div><div class="line">server向Client发送ACK</div><div class="line">15:55:29.810352 IP localhost.distinct &gt; localhost.56498: Flags 		[.], 	ack 101048876, win 12759, options [nop,nop,TS val 		726072074 ecr 	726072074], length 0</div></pre></td></tr></table></figure>
<p>  上面的抓包显示之间共有四次握手，其中最后一次server向client发送ack，在很多系统中是没有的，因为在第二步server已经向client发送了ack。（原本以为Macos没有使用tcp_delayed_ack,最后发现其实是使用了，不了解为什么会ack两次）。</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">mac 查询是否使用了tcp_delayed_ack</div><div class="line">终端执行：sudo sysctl -a | grep net.inet.tcp.delayed_ack</div><div class="line">net.inet.tcp.delayed_ack: 3</div><div class="line"></div><div class="line">delayed_ack=0 responds after every packet (OFF)</div><div class="line">delayed_ack=1 always employs delayed ack, 6 packets can get 1 		ack</div><div class="line">delayed_ack=2 immediate ack after 2nd packet, 2 packets per ack 		(Compatibility Mode)</div><div class="line">delayed_ack=3 should auto detect when to employ delayed ack, 4 		packets per ack. (DEFAULT recommended)</div></pre></td></tr></table></figure>
</li>
<li><p>4：client终端断开连接(Ctrl+C)</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div><div class="line">9</div><div class="line">10</div><div class="line">11</div><div class="line">12</div></pre></td><td class="code"><pre><div class="line">client向server发送FIN和seq</div><div class="line"></div><div class="line">15:41:51.867597 IP localhost.54819 &gt; localhost.distinct: Flags [F.], seq 3758577129, ack 382273321, win 12759, options [nop,nop,TS val 		725256837 ecr 725129655], length 0</div><div class="line">              		</div><div class="line">      server向client发送ACK</div><div class="line">      15:41:51.867652 IP localhost.distinct &gt; localhost.54819: Flags [.], ack 3758577130, win 12759, options [nop,nop,TS val 725256837 ecr 		725256837], length 0</div><div class="line">              		</div><div class="line">      server向client发送FIN+ACK（其实ACK上一步已经发送过了，有的系统只会发送一次，可能是网络优化）</div><div class="line">      15:41:51.867688 IP localhost.distinct &gt; localhost.54819: Flags [F.], seq 382273321, ack 3758577130, win 12759, options [nop,nop,TS val 		725256837 ecr 725256837], length 0</div><div class="line">              		</div><div class="line">      client向server发送ACK</div><div class="line">      15:41:51.867724 IP localhost.54819 &gt; localhost.distinct: Flags [.], ack 382273322, win 12759, options [nop,nop,TS val 725256837 ecr 		725256837], length 0</div></pre></td></tr></table></figure>
<p>  上面的四次握手和书中写的基本一致，但是在有的系统中第二步和第三步是作为一步的，即<code>server向client发送FIN+ACK</code>,可能和TCP_DELAYED_ACK有关系。</p>
</li>
</ul>
</li>
</ul>
<h3 id="测试二"><a href="#测试二" class="headerlink" title="测试二"></a>测试二</h3><ul>
<li><p>环境</p>
<ul>
<li>client server 同台机器</li>
<li>centos 7.2.1511</li>
</ul>
</li>
<li><p>执行</p>
<ul>
<li>1：终端执行 nc -l 9999 (模拟Server)</li>
<li>2：终端执行 tcpdump ‘port 9999’ -i lo -S (注意要找到ip为127.0.0.1对应的网卡)</li>
<li><p>3：终端执行nc localhost 9999 (模拟Client)</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">client向server发送SYN和seq</div><div class="line">15:47:53.855406 IP localhost.47686 &gt; localhost.distinct: Flags 		[S], seq 3848885127, win 43690, options [mss 65495,sackOK,TS val 		3471073701 ecr 0,nop,wscale 7], length 0</div><div class="line"></div><div class="line">server向client发送SYN+ACK和seq</div><div class="line">15:07:31.131256 IP localhost.distinct &gt; localhost.47686: Flags 		[S.], seq 2504659360, ack 3848885128, win 43690, options [mss 		65495,sackOK,TS val 3471073701 ecr 3471073701,nop,wscale 7], 		length 0</div><div class="line"></div><div class="line">client向server发送ACK</div><div class="line">15:47:53.855430 IP localhost.47686 &gt; localhost.distinct: Flags 		[.], ack 2504659361, win 342, options [nop,nop,TS val 3471073701 		ecr 3471073701], length 0</div></pre></td></tr></table></figure>
<p>  上面的抓包显示之间共有三次握手，区别上个测试，Server将SYN+ACK发送后，不会再重复发送ACK</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">mac 查询是否使用了tcp_delayed_ack</div><div class="line">终端执行：sudo sysctl -a | grep net.inet.tcp.delayed_ack</div><div class="line">net.inet.tcp.delayed_ack: 3</div><div class="line"></div><div class="line">delayed_ack=0 responds after every packet (OFF)</div><div class="line">delayed_ack=1 always employs delayed ack, 6 packets can get 1 		ack</div><div class="line">delayed_ack=2 immediate ack after 2nd packet, 2 packets per ack 		(Compatibility Mode)</div><div class="line">delayed_ack=3 should auto detect when to employ delayed ack, 4 		packets per ack. (DEFAULT recommended)</div></pre></td></tr></table></figure>
</li>
<li><p>4：client终端断开连接(Ctrl+C)</p>
  <figure class="highlight plain"><table><tr><td class="gutter"><pre><div class="line">1</div><div class="line">2</div><div class="line">3</div><div class="line">4</div><div class="line">5</div><div class="line">6</div><div class="line">7</div><div class="line">8</div></pre></td><td class="code"><pre><div class="line">client向server发送FIN和seq</div><div class="line">17:12:27.911405 IP localhost.47688 &gt; localhost.distinct: Flags 		[F.], seq 1376424971, ack 2766855978, win 342, options 		[nop,nop,TS val 3476147757 ecr 3476140792], length 0</div><div class="line">                </div><div class="line">server向client发送FIN+ACK</div><div class="line">17:12:27.911484 IP localhost.distinct &gt; localhost.47688: Flags 		[F.], seq 2766855978, ack 1376424972, win 342, options 		[nop,nop,TS val 3476147757 ecr 3476147757], length 0</div><div class="line">                		</div><div class="line">client向server发送ACK</div><div class="line">17:12:27.911493 IP localhost.47688 &gt; localhost.distinct: Flags 		[.], ack 2766855979, win 342, options [nop,nop,TS val 3476147757 		ecr 3476147757], length 0</div></pre></td></tr></table></figure>
<p>  上面的三次挥手和理论上的四次挥手又不太一致，本系统中将ACK+FIN一次发送,可能和TCP_DELAYED_ACK有关系。</p>
</li>
</ul>
</li>
</ul>
<h3 id="测试三"><a href="#测试三" class="headerlink" title="测试三"></a>测试三</h3><ul>
<li><p>环境</p>
<ul>
<li>clien server 不同机器</li>
<li>client macOS</li>
<li>server centos</li>
</ul>
</li>
<li><p>结论<br>  基本上和上面一样，建立连接三次握手，中断连接三次挥手</p>
</li>
</ul>

      
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